This invention relates to a method for the production of monoethylene glycol. More particularly, it relates to the improvement of a method in which monoethylene glycol is selectively produced from ethylene oxide via ethylene carbonate.
Monoethylene glycol is broadly used in polyester, polyurethane, non-freezing solution, cellophane, unsaturated polyester and the like.
Production of monoethylene glycol from ethylene oxide is generally carried out by hydrating to ethylene oxide in the absence of catalyst. The thus obtained hydration product is concentrated and then completely dehydrated by rectification while separating byproducts to obtain high purity monoethylene glycol.
In the hydration process of ethylene oxide, successive reaction of unreacted ethylene oxide with formed monoethylene glycol occurs and, as the result, diethylene glycol, triethylene glycol and more higher polyglycols are formed as by-products in addition to monoethylene glycol. In order to obtain monoethylene glycol most in demand with a high yield, it is necessary to supply large excess of water to reduce the successive reaction, generally from 10 to 25 moles of water based on 1 mole of ethylene oxide.
However, the addition of excess water for the purpose of improving the yield causes dilution of product in the formed solution, so that a large quantity of energy is required for removing the excess water in the distillation step.
In addition, the yield of monoethylene glycol obtained in this manner is merely around 90% which is not satisfactory.
In order to avoid such problems, a method has been proposed (JP-A-54-98765 (U.S. Pat. No. 4,314,945); the term "JP-A" as used herein means an "unexamined published Japanese patent application") in which monoethylene glycol is selectively obtained by producing ethylene carbonate from ethylene oxide and carbon dioxide (JP-A-57-31682) and then hydrolyzing ethylene carbonate (JP-A-55-154928 (U.S. Pat. No. 4,283,580)).
An advantage of this method is that the side reaction which generates diethylene glycol from monoethylene glycol and ethylene oxide does not occur and monoethylene glycol therefore is obtained with a markedly high yield, because the reaction for the formation of monoethylene glycol is carried out after ethylene oxide is once converted completely into ethylene carbonate.
However, the reaction for the production of ethylene carbonate from ethylene oxide is slow and requires isolation of ethylene oxide prior to the reaction.
As reactors for use in the production of the intermediate product ethylene carbonate, serial arrangement of a plurality of tube type reactors and condensers (Springmann, Fette Seifen Anstrichmittel, 73, 394-399 (1971)), a loop type reactor (Peppel, Industrial and Engineering, 50, 767-770 (1958)) and a bubble column reactor(JP-A-6-345699 (U.S. Pat. No. 5,508,442)) have been proposed.
Another method for the production of monoethylene glycol has also been proposed in which ethylene carbonate is used as the intermediate and the reaction is carried out by allowing water to coexist in advance in the reaction system [JP-A-54-19905 (U.S. Pat. No. 4,160,116), JP-A-49-86308 (U.S. Pat. No. 3,922,314), JP-B-49-24448 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-55-145623]. In this method, a mixture of ethylene carbonate and monoethylene glycol is first obtained by allowing ethylene oxide to react with carbon dioxide in water and then the remaining ethylene carbonate is completely hydrolyzed, so that monoethylene glycol as the product of interest can be obtained with a high yield without supplying excess amount of water. When water is present in this manner, the ethylene carbonation can be carried out markedly quickly and thus industrially advantageously, and water-containing ethylene oxide can be used as the material as it is.
Since hydrolysis of ethylene carbonate as the subsequent step is also carried out simultaneously in this method, load of the hydrolysis step can be reduced and the process therefore can be minimized.
In addition, the process can be simplified by directly combining it with an ethylene oxide production process. In general, production of ethylene oxide is carried out by the following method. Firstly, ethylene is converted into ethylene oxide by subjecting it to gas phase catalytic oxidation with oxygen in the presence of a silver catalyst, the thus obtained ethylene oxide-containing gas is allowed to contact with a large volume of water to effect absorption of ethylene oxide contained in the reaction gas by water and then ethylene oxide is recovered in the form of aqueous solution. Next, the thus obtained dilute aqueous solution of ethylene oxide (the ethylene oxide concentration is generally from 1 to 5% by weight) is heated under a reduced pressure to effect stripping and separation of ethylene oxide from the aqueous solution and then ethylene oxide is recovered from the top part of the reaction column. The absorbing water after the removal of ethylene oxide is cooled and again recycled to the absorption treatment. Thereafter, the aqueous mixture containing ethylene oxide as the main component thus obtained by the stripping treatment is subjected to distillation to remove water and thereby isolating and purifying ethylene oxide.
An important point of this method is to carry out the ethylene carbonate formation sufficiently faster than the side reaction which forms diethylene glycol from ethylene oxide and monoethylene glycol. For this purpose, it is essential to supply carbon dioxide to the liquid phase quickly and sufficiently in carrying out this reaction. However, nothing is known about a reactor in which the supply of carbon dioxide can be carried out sufficiently quickly and industrially safely and efficiently.
In addition, this reaction for producing ethylene carbonate from ethylene oxide generates considerably large heat of reaction so that it easily causes run away reaction. As a matter of course, the run away reaction is a problem to be avoided by all means, because it causes decomposition of catalyst and increases formation of aldehyde and the similar by-products, thereby exerting sheer adverse influences upon the quality of the ethylene glycol product.
In the case of the multiple pipe type reactors which are frequently used for this purpose, the reaction is not stable because of the difficulty in supplying each reaction pipe with carbon dioxide uniformly and of the inability to sufficiently cool the gas phase inside the pipes due to separation of carbon dioxide and reaction solution therein, so that the reaction sometimes does not progress smoothly and sometimes progresses too rapidly to cause run away reaction. A vessel type reactor equipped with an agitator requires considerable agitation power which still cannot dissolve carbon dioxide sufficiently in some cases. In addition, this type of reactor is not desirable in treating the toxic and explosive ethylene oxide from the viewpoint of safety, because its bearing part has a rolling friction which possibly causes leakage of gas or combusting and explosive reactions of ethylene oxide due to the frictional heat.
An object of the present invention is to provide a method for carrying out the reaction for obtaining a mixture of ethylene carbonate and monoethylene glycol safely and efficiently, in the process of selectively producing monoethylene glycol by allowing ethylene oxide to react with carbon dioxide in the presence of a carbonation catalyst and water, thereby obtaining said mixture, and then hydrolyzing the remaining ethylene carbonate.
Taking such actual circumstances into consideration, the inventors of the present invention have conducted intensive studies and found as a result of the efforts that stable reactions can be carried out by the use of a bubble column reactor, without accumulation of the heat of reaction or run away reaction. The present invention has been accomplished on the basis of this finding.